BMI, or Body Mass Index, is one of the most widely used tools in the world for assessing whether a person has a healthy body weight relative to their height. Doctors, health organisations, and researchers use it daily — yet many people are unsure exactly what it measures and what its limitations are.
This guide explains what BMI is, how it is calculated, what the different ranges mean, and when BMI may not tell the full picture.
BMI is a simple numerical value derived from a person's weight and height. It does not directly measure body fat, but it serves as a useful screening indicator to identify individuals who may be underweight, at a healthy weight, overweight, or obese.
The concept was developed in the 1830s by Belgian mathematician Adolphe Quetelet and later adopted by the World Health Organization (WHO) as a standard population health metric. Today, it remains one of the most practical tools available for initial weight assessment because it requires only two easily measured values: weight and height.
The formula is straightforward and the same for both men and women:
A person who weighs 70 kg and is 1.75 m tall:
A BMI of 22.9 falls within the normal weight range.
The World Health Organization classifies BMI into the following categories for adults aged 18 and over:
| Category | BMI Range | Health Implication |
|---|---|---|
| Underweight | Below 18.5 | Possible nutritional deficiency |
| Normal weight | 18.5 – 24.9 | Generally associated with good health |
| Overweight | 25.0 – 29.9 | Increased risk of some conditions |
| Obese (Class I) | 30.0 – 34.9 | High risk of chronic disease |
| Obese (Class II) | 35.0 – 39.9 | Very high risk |
| Obese (Class III) | 40.0 and above | Extremely high risk |
Note: These ranges apply to adults aged 18 and over. Children and teenagers use age- and sex-specific BMI percentile charts, as body composition changes significantly during growth.
A higher BMI, particularly in the overweight and obese ranges, is associated with an increased risk of several serious health conditions, including:
It is important to understand that BMI is a risk indicator, not a diagnosis. Many other factors — such as diet, physical activity, genetics, and where body fat is stored — also influence health outcomes.
While BMI is a useful population-level screening tool, it has well-recognised limitations when applied to individuals:
BMI cannot tell the difference between lean muscle mass and body fat. A professional athlete or bodybuilder may have a BMI in the overweight range despite having very low body fat. Conversely, an older person with low muscle mass may have a "normal" BMI but carry a higher proportion of body fat than is healthy.
Where fat is stored in the body matters significantly for health risk. Abdominal (visceral) fat — stored around the waist and organs — carries a higher health risk than fat stored in the hips and thighs. BMI gives no information about fat distribution. Waist circumference measurements can complement BMI for a more complete picture.
Research shows that people of Asian descent tend to have higher levels of body fat at lower BMI values compared to people of European descent. Many health organisations now recommend lower BMI thresholds for Asian populations — for example, a BMI of 23 or above may be considered overweight for people of East or South Asian heritage.
For those under 18, BMI must be interpreted using age- and sex-specific growth charts, as normal body composition changes considerably during childhood and adolescence.
Important: BMI is a screening tool, not a diagnostic instrument. Always consult a qualified healthcare professional for a complete and personalised health assessment.
Not necessarily. A person can have a BMI within the normal range and still have metabolic risk factors such as high blood pressure, elevated blood sugar, or unfavourable cholesterol levels. This is sometimes referred to as "metabolically obese, normal weight" (MONW).
On the other hand, some people with a BMI slightly above the normal range may have excellent metabolic health, particularly if they are physically active and have healthy blood markers.
BMI works best as one data point among many, not as a standalone measure of health.
If your BMI falls outside the healthy range, there are evidence-based strategies that can help:
Yes — the BMI formula and the standard WHO category ranges are the same for adult men and women. However, women naturally tend to have a higher percentage of body fat than men at the same BMI, which is worth bearing in mind when interpreting results.
For most adults, checking BMI once or twice a year is sufficient unless you are actively managing your weight, in which case monthly tracking may be more useful. Daily fluctuations in weight are normal and largely reflect water retention rather than true changes in body composition.
BMI is not an appropriate measure during pregnancy, as weight gain is expected and necessary. Pregnant women should follow the guidance of their midwife or doctor regarding healthy weight gain during pregnancy.
For adults aged 18 to 65, the standard WHO healthy BMI range of 18.5 to 24.9 applies. For adults over 65, some research suggests that a slightly higher BMI (around 22 to 27) may be associated with better health outcomes, as lower BMI in older adults can sometimes indicate muscle loss or underlying illness.
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